Since materials with a porous structure have a large surface area, they have been extensively used as a carrier for catalysts and a carrier for fixedly supporting enzymes or functional organic compounds thereon. In particular, the materials with a porous structure having a sharp pore size distribution exhibit a function as a molecular sieve and are therefore usable in the applications such as a carrier for catalysts having a structural selectivity or a separating agent for various materials. In such applications, there is a demand for porous materials having uniform and fine pores.
As the porous materials having uniform and fine pores, mesoporous silica having meso-order pores has been developed. It has been noticed that the mesoporous silica is used in the applications such as nanowires, semiconductors and optical electronics in addition to those described previously.
As the silica having a mesoporous structure, there are known silica particles having an outer shell of a mesoporous structure and a hollow inside portion. For example, Patent Document 1 discloses a process for producing a hollow silica microcapsule having a mesoporous wall in which mesopore-free hollow silica particles produced using emulsified droplets of an organic solvent are subjected to high-temperature heat treatment in the presence of a surfactant to form mesopores therein. It is also described that the mesoporous wall is utilized for encapsulating agricultural chemicals, medicines or drugs, cosmetics, aromatizing agents, etc., therein. However, the makeup tests actually made by the inventors failed to obtain mesoporous silica having a hollow structure, and resulted only in production of a mixture of mesopore-free hollow silica particles, solid silica particles and amorphous mesoporous silica particles having not hollow structure.
Patent Document 2 discloses a composite porous material in the form of mesoporous silica particles containing an organic group in which pores involved in 60% or more of a whole pore volume of the silica particles lie within a range of ±40% of a pore size distribution having a maximum peak. In the process for producing the mesoporous silica particles as described in the Patent Document 2, for example, tetramethoxysilane is used in combination with bis(trimethoxysilyl)methane. However, both these silane materials have high hydrolysis rates which are not largely different from each other, thereby failing to produce hollow particles.
Non-Patent Documents 1 and 2 disclose hollow mesoporous silica particles produced using emulsified droplets of trimethyl benzene. However, since a neutral polymer is used as a mesoporous structure template, the resulting pore structure has a low regularity and a BET specific surface area as low as 430 m2/g.
The Non-Patent Document 2 also discloses the use of the hollow mesoporous silica particles and reported the results of study on diffusion of the silica particles into a dye aqueous solution by preparing silica particles in which dye-containing trimethyl benzene is incorporated. However, mesopores on the surface of the respective silica particles are filled with the neutral copolymer.
Non-Patent Documents 3 and 4 disclose hollow mesoporous silica particles which are synthesized by neutralizing the silica particles with an acid at an early reaction stage to interrupt the particle growth reaction. Therefore, the hollow mesoporous silica particles have a BET specific surface area as relatively high as from 850 to 950 m2/g, but exhibit a broad particle size distribution.
Non-Patent Document 5 discloses hollow mesoporous silica particles formed by irradiating an ultrasonic wave to a reaction solution. Therefore, the hollow mesoporous silica particles have a BET specific surface area as relatively high as 940 m2/g, but exhibit a very broad particle size distribution as well as an amorphous particle shape.
The silicon material used as a raw material in the Non-Patent Documents 1 and 2 is a water glass, whereas the silicon material used in the Non-Patent Documents 3 to 5 is tetraethoxysilane. Therefore, the silica particles produced from these silicon materials fail to have an outer shell containing an organic group.    Patent Document 1: JP 2006-102592A    Patent Document 2: JP 2000-219770A    Non-Patent Document 1: Qianyano Sun, et at, “Adv. Mater.”, Vol. 15, p. 1097 (2003)    Non-Patent Document 2: Nicole E. Botterhuis, et al., “Chem. Eur. J.”, Vol. 12, p. 1448 (2006)    Non-Patent Document 3: Puyam S. Singh, et al., “Chem. Lett.”, p. 101 (1998)    Non-Patent Document 4: Christabel E. Fowler, et al., “Chem. Commun.”, p. 2028 (2001)    Non-Patent Document 5: Rohit K. Rana, et al., “Adv. Mater.”, Vol. 14, p. 1414 (2002)